#20228 Added more aggresive tests and updated documentation.

[#20228](https://code.djangoproject.com/ticket/20228) seems to be valid documentation bug to me. Given example in ticket is trying to validate_unique while one of the field which is in constraint is not editable or in exclude making validate_unique skip validation. This is expected behavior but documentation doesn't explain this behavior. I have added:

* Note in documentation.
* More aggressive test cases.